Make extract_point/extract_location/extract_coordinate_points preprocessor functions lazy

[bouweandela]

These preprocessor functions depend on interpolation schemes in Iris:

ESMValCore/esmvalcore/preprocessor/_regrid.py

Lines 75 to 79 in d37a502

	# Supported point interpolation schemes.
	POINT_INTERPOLATION_SCHEMES = {
	'linear': Linear(extrapolation_mode='mask'),
	'nearest': Nearest(extrapolation_mode='mask'),
	}

which are not lazy:
https://github.com/SciTools/iris/blob/57096f29f6f2c5ff278d616df5e2da4d2bab95e2/lib/iris/analysis/_interpolation.py#L208-L209

[bouweandela]

@fnattino Is planning to look into this.